The procedures for spectrally sensitizing photographic silver halide emulsions so as to extend their sensitivity range are generally well known. In particular, for some Graphic Arts applications it may be necessary to sensitize the silver halide grains to specific wavelengths, for example to light of wavelength 633 nm for helium-neon gas laser (HN) or about 670 nm for laser diode (LD) and red light emitting diodes (LED) devices. There are numerous examples of suitable sensitizing dyes for these purposes, for example, see The Theory of the Photographic Process, T. H. James, editor, 4th edition, pages 195-234 for leading references. In British Patent Specification 1,471,701 is described the use of tri-nuclear cyanine dyes for such purposes.
However, in Graphic Arts applications, sensitizing dyes are also required to provide many other qualities, such as low post-process stain while still providing good speed. Post-process stain is distracting and is therefore considered undesirable. It can in some circumstances cause undesirably high minimum ultraviolet light density. Additionally, photographic elements for graphic arts applications may contain hydrazine nucleating agents and optionally also amine boosters. In such elements or processes, the sensitizing dyes used may adversely affect photographic parameters such as the effective speed of the emulsion or the level of nucleation.
The use of trinuclear merocyanine sensitizing dyes is described in U.S. Pat. No. 5,116,722, EP 0 540 295, U.S. Pat. No. 3,682,640 and USSR Inventor's Certificate No. 212,749. Specific tetranuclear merocyanine dyes for sensitizing photographic emulsions are described in GB 489,335.
However, it would be desirable to provide photographic elements which are processed in the presence of hydrazine nucleators and optionally amine boosters. The emulsions of such elements would be sensitized by dyes which contribute low stain following processing of the element, and which still provide good sensitization while having little or no adverse affect on other parameters such as the effective speed of the emulsion or increased minimum density ("Dmin").